It's the San Remo Roll into the Millennium error which was released in 1999, two years after it was required to be on the silver. There is a correct version of the Roll into the Millennium strike that has the .999 on the silver. I didn't put both sides on strikepoint, but if you look on eBay you will see the pics of both sides.

Also, so you know, if it wasn't really an error, I wouldn't say it was.

Dave.
SS - 1029